This invention relates generally to a radio frequency (RF) choke, and more specifically concerns a choke for separating an AC power wave from a broadband signal, where both are carried on the same conductors.
It is common in CATV distribution systems to use a broadband signal (e.g. 50 to 1000 MHZ) to carry the various channels and other information to the subscribers, another broadband signal (e.g. 5 to 40 MHZ) to carry information from the subscribers to the cable distribution station, and 60 Hz single phase power to operate amplifiers and other devices located at various points on the cable system. In such systems, the broadband signals and AC power are typically each carried on the same transmission line, e.g., the center conductor and braid of a coaxial cable.
Each of the broadband signals originate from a central location. The coaxial cables used to carry these signals inherently have loss characteristics. Thus, amplifier stations must be installed at appropriate locations along the cable in order to compensate for the losses and deliver faithful broadband (xe2x80x9cRFxe2x80x9d) signals. The single phase AC power signal is needed to operate the amplifier stations.
The power signal is passed along the cable concurrently with the RF signal. The power level of the AC signal is much greater than that of the RF signal, and uses different and separate circuitry to operate the amplifier station. Therefore, the AC power signal must be separated from the RF signal at each of the amplifier stations.
It is common practice to use an RF choke and a capacitor to separate the single-phase AC power signal from the broadband RF signals at points along the cable where the RF signal is to be processed in an RF device. After passing the device, the AC power is recombined with the broadband signal, requiring the use of a second RF choke.
The AC power has a current magnitude up to 15 amperes at 60 volts. On the other hand, the broadband RF signal has a low peak voltage of about 0.3 volts. When isolating the AC power from an RF device, the chokes must prevent the RF broadband signal from passing through the choke along the AC power path in order to avoid a significant loss of signal.
One choke, developed by the present inventor which solves many of the problems in the art, is described in U.S. Pat. No. 5,032,808 which is incorporated herein by reference. The choke described in the ""808 patent comprises a series of three sets of windings, each of which has a distinct number turns wound upon a core having a uniform cross sectional area along its entire length. The first set of windings is connected at one end to an input lead and at an opposite end to the second set of windings through an air coil inductor. The second set of windings is similarly connected to one end of a third set of windings by means of a second air coil inductor. The output end of the third set of windings in turn is connected to an output lead.
A commercial RF choke is typically constituted by a number of turns of insulated copper wire wound upon a ferrite coil form. A resistor can be connected in parallel with a portion of this wire coil, e.g., from a preselected turn to one of the lead wires, to serve as a shunt. This parallel resistor is selected so that it does not significantly reduce the impedance of the RF choke. There is an effective capacitance between turns of the wire coil, which produces a self-capacitance that combines with the coil inductance to produce an LC resonance. Typically, such resonances unfortunately often lie within the band of the broadband RF signal. The effect of the shunt resistance is to reduce the Q of the LC resonance, thereby blunting the sharpness of any in-band resonances.
A reduction in the number of turns of the wire coil can push any LC resonances above the passband, but this reduction will also result in a reduction in inductance, limiting the suitability of the choke at the 5 MHZ low end of the band. The presence of the shunt resistor in the above-described choke also reduces the signal impedance to ground, thereby increasing the signal loss.
In addition to the effects on frequency response, the RF chokes used in the equipment of the cable system must be capable of passing several amperes of AC current. The wire used for the coil must therefore be large enough to carry relatively high currents, usually up to 15 amperes in such cable transmission systems, without becoming excessively warm. Unfortunately, the larger the wire size the more troublesome the related parasitic resonance problem becomes. High currents also pose problems in that core materials are likely to approach saturation, thereby presenting the RF signals with an impedance which varies at the frequency rate of the single phase AC power signal. The effect of this is an unwanted modulation of RF signals commonly referred to as xe2x80x9chum mod.xe2x80x9d
The above described problems related to high AC current can be effectively reduced by careful selection of wire size, core material, core geometry, shunt resistors and winding dimensions. Many RF chokes have been used from give good performance to the 5 MHZ to 450 MHZ frequency range. However, when these chokes are used for the 5 MHZ to 1,000 MHZ frequency range, they exhibit a moderate amount of insertion losses at about 750 MHZ. High attenuation of the signal results when the losses are allowed to cascade over many circuits. It is desirable to maximize the reduction of insertion losses as many chokes are cascaded over large networks. Thus, savings of even the smallest amount of insertion loss manifests into a substantial amount of power savings over a large network.
Cable system capabilities are needed for extended bandwidths and upper frequency limits beyond 750 MHZ to 1 GHz and higher. Therefore, a need exists for an improved RF choke which overcomes the problems and shortcomings associated with the prior art.
In accordance with the present invention, an improved RF choke is disclosed for use in cable and telephone systems over which RF signals and AC power signals are each transmitted and distributed.
It is an object of this invention to provide an improved RF choke.
Another object of this invention is to separate RF signals from AC power signals on a cable network using a choke having reduced insertion loss as well as reduced hum modulation at radio frequencies of between about 5 MHZ and 1 GHz.
It is a further object of the invention to reduce the insertion loss of an RF choke to save substantial amount of power which is usually lost when circuits incorporating the RF choke are cascaded in a large network.
Generally speaking, the present invention comprises an RF choke that includes an elongated ferromagnetic core, usually but not necessarily composed of ferrite, having first and second end portions which have similar magnetization and saturation characteristics, a first winding which encircles the first end portion in a first direction and has a first predetermined number of turns, and second and third windings which encircle the second end portion in a second, opposite direction and have second and third predetermined numbers of turns, respectively. These three windings are connected in series with one another between a pair of leads that serve to connect them to the external circuitry with which the choke is used.
In accordance with a first important feature of the present invention, the number and spacing of the turns within the first winding are so related to the numbers and spacings of the turns within the second and third windings that relatively low frequency currents, such as AC power currents, generate substantially canceling fluxes in the first and second end portions of the core, while relatively high frequency currents, such as those used to transmit broadband RF signals, do not generate canceling fluxes in the first and second end portions of the core. This allows the choke to present a negligible or at least a relatively low inductance to relatively low frequency currents, while presenting a relatively higher inductance to relatively high frequency currents, thereby enhancing the ability of circuits which include the choke to discriminate between and separate the power and signal bearing components of the currents flowing therethrough.
In preferred embodiments of the present invention, the windings that are associated with the two end portions of the core are separated by a predetermined distance which is selected to enhance the frequency discriminating capabilities which the choke exhibits as a result of the above-mentioned relationships between the numbers and spacings of the turns of the windings. The effect of this separation or spacing between these windings is to alter the degree to which fluxes generated within the end portions of the core cancel one another, particularly at the high end of the range of frequencies over which the choke is designed to operate. In particular, higher separations or spacings have the effect of reducing the amount of flux cancellation which occur at the high end of this range, and thereby increasing the difference between the inductive impedances which the choke presents at high and low frequencies. In embodiments in which the diameters of the first and second end portions are different, this spacing is preferably approximately equal to the diameter of the smaller one of these end portions. In embodiments in which these diameters are the same, this spacing is approximately equal to this diameter.
Since cores of the type used in the present invention will ordinarily be molded under pressure, they will ordinarily have a small taper or draw, i.e., have diameters which vary slightly along the length thereof. In order to avoid repeatedly referring to this diameter variation in the description of the invention which follows, it will be understood that the term xe2x80x9cdiameterxe2x80x9d, as used herein, refers to a dimension of the core which is perpendicular to the longitudinal axis thereof and which is selected according to a suitable uniformly applied rule, such as the average diameter, the diameter at the smaller or larger end, etc., and encompasses all cores which comply with that rule, without regard to the magnitude of their taper or to the lack thereof.
The core used in the RF choke of the present invention may have end portions that take a variety of different forms and/or have a variety of different configurations. In a first type of embodiment, for example, the core may comprise a single piece of ferromagnetic material having end portions with substantially similar diameters, or two pieces of ferromagnetic material, referred to herein as core members, with substantially similar diameters that are secured together in end to end relationship with one another, with only a negligibly thin layer of glue disposed therebetween. Cores of this type may, on the other hand, comprise a single piece of magnetic material having end portions with unequal diameters, or two pieces of magnetic material with unequal diameters which are secured together.
In a second type of embodiment, one or both end portions of the core may comprise a core assembly made up of two or more pieces, herein referred to as core segments, which are disposed in end to end relationship with one another. Cores of this type have the advantage that they have the effect of spreading the taper that is associated with the use of molding processes along the length of the core as a whole, and thereby reducing the diameter differences that accumulate along the length thereof. Cores of this type will often be referred to herein as segmented or stacked cores. As in the case of cores which have end portions which are not segmented, cores which have end portions which are segmented may have end portions that have either equal or unequal diameters.
In still another type of embodiment, the end portions of the core may have different compositions, provided that they have similar magnetization and saturation characteristics, together with sizes that permit them to exhibit frequency dependent flux cancellation characteristics of the kind necessary for them to accomplish their purpose. An end section having a relatively large diameter and a relatively low permeability may, for example, be combined with an end section having a relatively small diameter and a relatively high permeability. Embodiments of this type may be implemented in any core having two or more pieces, including any of the core configurations described above.
Other combinations of core numbers, diameters and compositions will be apparent to those skilled in the art. It will be understood that all such combinations are within the contemplation of the present invention.
In addition to having any of the types of cores described above, the choke of the present invention may also have a core in which the two end portions are separated by an air gap which has a predetermine length, or by a non-magnetic spacing element that establishes and maintains a predetermined distance therebetween. The effect of such an air gap or spacer is generally similar to that produced by the above-described separation or spacing between the windings that are associated with the two end portions of the core, namely: to reduce the amount of flux cancellation which occurs at the high end of the range of frequencies over which the choke is used. While either an air gap or a spacing element may produce this effect, a spacing element is preferred because it can serve as a structure to which the two parts of the core may both be glued or otherwise attached, and thereby effectively maintained at the desired distance with a high degree of accuracy. Embodiments of this type may be implemented in any core having two or more pieces, including any of the core configurations described above.
Finally, preferred embodiments of the invention include a plurality of resistors which have the effect of flattening the inductance vs. frequency characteristics of the choke, particularly over parts or bands of its design frequency range where it tends to exhibit resonances. These resistors may be connected either between the leads of the choke and predetermined respective ones of the turns of any of the windings, or between predetermined ones of the turns of the windings themselves. The optimum numbers and connections of these resistors will ordinarily depend on the structure of cores and windings used in the choke, and will therefore be different depending on which of the above described embodiments is used.